The Relativistic Precession Model for QPOs in Low Mass X-ray Binaries

The relativistic precession model for quasi periodic oscillations, QPOs, in low mass X-ray binaries is reviewed. The behaviour of three simultaneous types of QPOs is well matched in terms of the fundamental frequencies for geodesic motion in the strong field of the accreting compact object for reasonable star masses and spin frequencies. The model ascribes the higher frequency kHz QPOs, the lower frequency kHz QPOs and the horizontal branch oscillations to the Keplerian, periastron precession and nodal precession frequencies of matter inhomogeneities orbiting close to the inner edge of the accretion disk. The remarkable correlation between the centroid frequency of QPOs in both neutron star and black hole candidate low mass X-ray binaries is very well fit by the model. QPOs from low mass X-ray binaries might provide an unprecedented laboratory to test general relativity in the strong field regime.Comment: 12 pages, 2 embedded figures, Proceedings of X-ray Astronomy '999 - Stellar Endpoints, AGN and the Diffuse Background, 2000. G. Malaguti, G. Palumbo & N. White (eds), Gordon & Breach (Singapore

Boundary Spatiotemporal Correlations in a Self-Organized Critical Model of Punctuated Equilibrium

In a semi-infinite geometry, a 1D, M-component model of biological evolution realizes microscopically an inhomogeneous branching process for $M \to \infty$. This implies in particular a size distribution exponent $\tau'=7/4$ for avalanches starting at a free end of the evolutionary chain. A bulk--like behavior with $\tau'=3/2$ is restored if `conservative' boundary conditions strictly fix to its critical, bulk value the average number of species directly involved in an evolutionary avalanche by the mutating species located at the chain end. A two-site correlation function exponent ${\tau_R}'=4$ is also calculated exactly in the `dissipative' case, when one of the points is at the border. These results, together with accurate numerical determinations of the time recurrence exponent $\tau_{first}'$, show also that, no matter whether dissipation is present or not, boundary avalanches have the same space and time fractal dimensions as in the bulk, and their distribution exponents obey the basic scaling laws holding there.Comment: 5 pages, 3 eps figure

Branching Interfaces with Infinitely Strong Couplings

A hierarchical froth model of the interface of a random $q$-state Potts ferromagnet in $2D$ is studied by recursive methods. A fraction $p$ of the nearest neighbour bonds is made inaccessible to domain walls by infinitely strong ferromagnetic couplings. Energetic and geometric scaling properties of the interface are controlled by zero temperature fixed distributions. For $p<p_c$, the directed percolation threshold, the interface behaves as for $p=0$, and scaling supports random Ising ($q=2$) critical behavior for all $q$'s. At $p=p_c$ three regimes are obtained for different ratios of ferro vs. antiferromagnetic couplings. With rates above a threshold value the interface is linear ( fractal dimension $d_f=1$) and its energy fluctuations, $\Delta E$ scale with length as $\Delta E\propto L^{\omega}$, with $\omega\simeq 0.48$. When the threshold is reached the interface branches at all scales and is fractal ($d_f\simeq 1.046$) with $\omega_c \simeq 0.51$. Thus, at $p_c$, dilution modifies both low temperature interfacial properties and critical scaling. Below threshold the interface becomes a probe of the backbone geometry (\df\simeq{\bar d}\simeq 1.305; $\bar d$ = backbone fractal dimension ), which even controls energy fluctuations ($\omega\simeq d_f\simeq\bar d$). Numerical determinations of directed percolation exponents on diamond hierarchical lattice are also presented.Comment: 16 pages, 3 Postscript figure

Pseudoknots in a Homopolymer

After a discussion of the definition and number of pseudoknots, we reconsider the self-attracting homopolymer paying particular attention to the scaling of the number of pseudoknots at different temperature regimes in two and three dimensions. Although the total number of pseudoknots is extensive at all temperatures, we find that the number of pseudoknots forming between the two halves of the chain diverges logarithmically at (in both dimensions) and below (in 2d only) the theta-temparature. We later introduce a simple model that is sensitive to pseudoknot formation during collapse. The resulting phase diagram involves swollen, branched and collapsed homopolymer phases with transitions between each pair.Comment: submitted to PR

Scaling and efficiency determine the irreversible evolution of a market

In setting up a stochastic description of the time evolution of a financial index, the challenge consists in devising a model compatible with all stylized facts emerging from the analysis of financial time series and providing a reliable basis for simulating such series. Based on constraints imposed by market efficiency and on an inhomogeneous-time generalization of standard simple scaling, we propose an analytical model which accounts simultaneously for empirical results like the linear decorrelation of successive returns, the power law dependence on time of the volatility autocorrelation function, and the multiscaling associated to this dependence. In addition, our approach gives a justification and a quantitative assessment of the irreversible character of the index dynamics. This irreversibility enters as a key ingredient in a novel simulation strategy of index evolution which demonstrates the predictive potential of the model.Comment: 5 pages, 4 figure

The Discovery of 8.7s Pulsations from the Ultrasoft X-Ray Source 4u0142+614

We discovered a periodicity at about 8.7s from the X--ray sources 4U0142+61, previously considered a possible black hole candidate on the basis of its ultrasoft spectrum. The pulsations are visible only in the 1--4 keV energy range, during an observation obtained with the EXOSAT satellite in August 1984. A search for delays in the pulse arrival times caused by orbital motion gave negative results. In the same data, periodic oscillations at 25 minutes had been previously found in an additional hard spectral component dominating above 4 keV which arises from the X-ray transient RX J0146.9+6121, discovered with ROSAT and identified with a Be star. Though the very high (>10^4) X--ray to optical flux ratio of 4U0142+61 is compatible with models based on an isolated neutron star, the simplest explanation involves a low mass X--ray binary with a very faint companion, similar to 4U1626--67. The discovery of periodic pulsations from 4U0142+61 weakens the phenomenological criterion that an ultrasoft spectral component is a signature of accreting black holes.Comment: plain LaTeX v3.1, 14 pages + 2 PostScript figures available upon request to [email protected] . To appear on The Astrophysical Journal, Letters. SISSA ref.: 106/94/

Strong gravitational field light deflection in binary systems containing a collapsed star

Large light deflection angles are produced in the strong gravitational field regions around neutron stars and black holes. In the case of binary systems, part of the photons emitted from the companion star towards the collapsed object are expected to be deflected in the direction of the earth. Based on a semi-classical approach we calculate the characteristic time delays and frequency shifts of these photons as a function of the binary orbital phase. The intensity of the strongly deflected light rays is reduced by many orders of magnitude, therefore making the observations of this phenomenon extremely difficult. Relativistic binary systems containing a radio pulsar and a collapsed object are the best available candidates for the detection of the strongly deflected photons. Based on the accurate knowledge of their orbital parameters, these systems allow to predict accurately the delays of the pulses along the highly deflected path, such that the sensitivity to very weak signals can be substantially improved through coherent summation over long time intervals. We discuss in detail the cases of PSR 1913+16 and PSR 1534+12 and find that the system geometry is far more promising for the latter. The observation of the highly deflected photons can provide a test of general relativity in an unprecedented strong field regime as well as a tight constraint on the radius of the collapsed object.Comment: 7 pages, uuencoded, gzip'ed, postscript file with figures included. Accepted for pubblication in MNRA

On the anomalous X-ray afterglows of GRB 970508 and GRB 970828

Recently, BeppoSAX and ASCA have reported an unusual resurgence of soft X-ray emission during the afterglows of GRB 970508 and GRB 970828, together with marginal evidence for the existence of Fe-lines in both objects. We consider the implications of the existence of a torus of iron-rich material surrounding the sites of gamma ray bursts as would be expected in the SupraNova model; in particular, we show that the fireball will quickly hit this torus, and bring it to a temperature ~3x10^7 K. Bremsstrahlung emission from the heated up torus will cause a resurgence of the soft X-ray emission with all expected characteristics (flux level, duration and spectral hardening with time) identical to those observed during the reburst. Also, thermal emission from the torus will account for the observed iron line flux. These events are also observable, for instance by new missions such as SWIFT, when beaming away from our line sight makes us miss the main burst, as Fast (soft) X-ray Transients, with durations ~10^3 s, and fluences ~10^-7-10^-4 erg cm^-2. This model provides evidence in favor of the SupraNova model for Gamma Ray Bursts.Comment: To appear in MN Pink pages, MN-LateX, no figure